This invention relates to electron gun assemblies and, more particularly, to three gun assemblies for color television picture tubes.
Electron gun assemblies to which this invention relates comprise, generally, a battery of three guns, a pole piece convergence assembly, and a glass stem provided with rigid metal prongs or leads. Each of the guns includes a plurality of electrode elements, usually cylindrically shaped, and positioned along a common axis. Electron guns may be placed in two categories, delta and in-line, depending upon the orientation of the three guns. The in-line type of gun, to which this invention relates, may be defined as the type wherein the longitudinal axes of the three guns lie in the same plane. With this type of gun the three dots produced thereby appear on the cathode ray tube screen in a line as opposed to a triad as is the case with the delta gun.
A precise degree of alignment accuracy is required with respect to the supporting arrangement for the three guns as well as to the connection of the stem to the gun assembly and the convergence assembly to the guns. As higher resolution is required in cathode ray tubes, the size of the phosphor dot in the screen must be smaller. The number of dots must also be increased. This smaller dot size, in turn, requires better landing accuracy of the electron spot which provides a lower tolerance for errors caused by poor electron gun alignment. Therefore, with poor alignment the beam may be displaced too far from its desired point of landing on the screen. Errors in the linear direction of the outside beams, that is, may be compensated for by means of the pole pieces in the convergence assembly. Errors in the center beam are compensated for by a purity magnet. Rotational errors, on the other hand, in the in-line tube, cannot be compensated for. Thus, if the entire gun assembly is rotated, the line on the screen in which the three spots appear would be canted in relation to the axes of the tube which is not tolerable.
The causes of poor alignment arise at various parts in the entire gun assembly. The battery of electron guns is usually mounted to support rods, commonly made of an electrically insulating material such as glass and designated as beads. In the in-line type of gun, the need for a rigid mounting structure is critical because the two outer guns are referenced to the center gun and there is no control over the center gun beam through the convergence assembly. With prior art mounting arrangements, the guns often move out of alignment during the manufacture and assembly of the rest of the structure. The connection of the stem to the gun assembly is also a source of alignment problems. In the prior art arrangements, the leads or prongs on the stems have been attached to the gun elements, and a force applied on the stem will necessarily be applied to the gun elements and can move them out of alignment. Moreover, in these designs the heaters and cathodes of the gun elements are connected directly to the stem and it was thus necessary to put the stem on first. The cathode and heater elements were thus difficult to get at during assembly, the structure could be moved out of alignment in trying to do so, and automated assembly was thus virtually impossible.
The registering of the pole piece convergence assembly to the rest of the gun assembly has also been a source of misalignment. Arrangements designed heretofore for mating the convergence assembly to the gun assembly have required some sort of fitting-in of one part to another as, for example, where detents are put in the base element of the convergence assembly. With the in-line arrangement an even more accurate alignment is needed and it would be desirable to have a self-registering alignment scheme to provide maximum protection against rotational errors. Finally, the assembling of the pole piece elements within the convergence assembly must be done accurately, and this is especially critical in the in-line type gun because, in order to get the proper raster size, the center gun must be magnetically isolated. Again, a self-registering arrangement would be most desirable.